The complement system comprises a group of plasma proteins which interact sequentially to effect certain immunologic reactions. Elucidation of the sequence and mechanisms of the classical complement system has been facilitated by the use of "cellular intermediates" formed by the controlled assembly of active complement enzymes or complexes on the membranes of sheep erythrocytes coated with rabbit antibody. Concepts derived from the interaction of defined cellular intermediates with purified components of the classical hemolytic complement sequence will be extended to the study of alternative pathways of complement activation, whose mechanisms of action have not yet been clarified. Interaction of purified properdin system proteins with sheep erythrocytes coated with C3b (EAC43) or with bacterial lipopolysaccharide (LPS-E) or rabbit erythrocytes in the formation of hemolytic intermediates of the properdin pathway may permit the sequential analyses and sensitive assays required to unravel the reaction mechanisms of this system. Using techniques adapted from the immune hemolytic system, the effects of assembling complement or properdin enzyme complexes on the surfaces of non-erythrocytic cells will also be investigated. Receptors for IgG-Fc, C4b, C3b or C56 may mediate the binding of such enzyme complexes to membranes of polymorphonuclear leukocytes, mast cells, macrophages and platelets, whose perturbation by plasma protein systems may have greater immunopathologic consequences than classical immune hemolysis.